The present invention generally relates to a process for the production of microcapsules which may be used to coat carbonless copy papers. More particularly, the present invention relates to a process for making microcapsules where the wall material is the reaction product of a tertiary aliphatic isocyanate and an amine-containing composition that is at least trifunctional in primary amine groups.
An the manufacture of pressure-sensitive recording papers, better known as carbonless copy papers, a layer of pressure-rupturable microcapsules containing a solution of colorless dyestuff precursor is normally coated on the back side of the front sheet of paper of a carbonless copy paper set. This coated back side is known as the CB coating. In order to develop an image or copy, the CB coating must be mated with a paper containing a coating of a suitable color developer, also known as dyestuff acceptor, on its front. This coated front color developer coating is called the CF coating. The color developer is a material, usually acidic, capable of forming the color of the dyestuff by reaction with the dyestuff precursor.
Marking of the pressure-sensitive recording papers is effected by rupturing the capsules in the CB coating by means of pressure to cause the dyestuff precursor solution to be exuded onto the front of the mated sheet below it. The colorless or slightly colored dyestuff precursor then reacts with the color developer in the areas at which pressure was applied, thereby effecting the colored marking. Such a mechanism for the technique of producing pressure-sensitive recording papers is well known.
Also well known are self-contained (SC) sheets which have the CB coating and the CF coating layered or admixed on a support sheet. Such sheets are also considered to be carbonless copy papers.
The microcapsules used in carbonless copy paper systems generally contain an oleophilic core material containing a dyestuff precursor encapsulated within strong protective outer walls or shells. The wall or shell acts to retain the core material until the wall is ruptured by mechanical pressure. Typically, microcapsule slurries used to coat the copy papers are comprised of individual capsules which may range in size from 0.1 to 50 microns in size.
The encapsulation of an oily solution by dispersing or emulsifying a wall-forming material and a polyamine or polyamine adduct in an aqueous solution is known in the art. For example, U.S. Pat. No. 4,021,595 teaches the use of a polyisocyanate adduct having a free isocyanate group and a polyamine or polyamine adduct having a free amine group as a polymerization promoter, where the polyisocyanate is dissolved in an oily liquid, wherein the oily liquid is dispersed or emulsified in a polar solvent. The addition of the polyamine having a free amine group to the emulsion or dispersion insolubilizes the polyisocyanate at the interface. This produces microcapsules for use on pressure-sensitive recording papers.
In U.S. Pat. 4,193,889, a film-forming aliphatic polyisocyanate containing at least one biuret group with a chain-extending agent is dissolved in an oily liquid and the encapsulation is carried out by reaction at the organic interface. The oily solution is dispersed under high shear in an aqueous phase containing a polyamine which is capable of reacting with isocyanates. The preferred polyisocyanate used in the process is based on hexamethylene 1,6-diisocyanate that forms a microcapsule wall when reacted in the aforementioned manner.
Similarly, in U.S. Pat. No. 4,428,978, microcapsules are produced by interfacial polyaddition from polyisocyanate and components containing active hydrogen. The isocyanurate-modified aliphatic polyisocyanate is reacted with a primary or secondary polyamine and the resulting suspension is reduced to a pH of 7 or below immediately after the polyaddition reaction. The slightly acidic pH of the resulting suspension slows the rate of reaction between the isocyanate and amine to produce a fully crosslinked capsule wall. Examples of primary and secondary amines used include diethylenetriamine and (bis(3-aminopropyl) amine.
The prior art references use diisocyanates coupled with either primary or secondary amines. However, it would be desirable to be able to use alternative materials to form capsule walls which are strong and retain the core material until ruptured. Accordingly, there is a need for a process for producing pressure-rupturable microcapsules by reacting alternative capsule wall materials to produce microcapsule walls which are strong enough to hold the microcapsule contents over an extended period of time and are rupturable only upon application of mechanical pressure.